A new homopolynucleotide triple helix poly dt. 2 poly dA has been discovered and characterized. Detected by CD mixing curves, the complex is the product of an exothermic reaction of dA.dT with dA at moderate temperature and high salt concentration. At intermediate temperature the double helix undergoes a novel double disproportionation reaction to form both the new triple helix and the familiar dA.2dT. Differential scanning calorimetry was used to characterize the system thermodynamically. Complete phase diagrams were determined in NaC1, NaBr, and NaC10. We had previously obtained the first crystals of DNA triple helices and observed fiber-type X-ray diffraction rather than the expected crystal diffraction. We have refined the molecular modeling to obtain good agreement with Fourier transforms calculated from the X-ray data, providing experimental support for the model. Helical parameters and coordinates were obtained for two distinct triple helices. A search of numerous sequence to find a triple helix suitable for a crystallographic structure determination produced only crystals that diffracted like fibers, suggesting that this behavior is an intrinsic property of triplexes. We have therefore investigated mixed structures and have now obtained normal crystal-type X-ray diffraction from DNA sequences containing both triple and double helical portions. Diffraction is observed to 2.8 A. Dr. Davies and members of his group have begun efforts to solve the structure.